I always offloaded the batch-work; data entry that could be shown once and might be remembered next month/quarter/year.
That's what I was thinking. Get someone to do the straight data entry, review it, and do the high level stuff yourself. Another option is to find a good bookkeeper who's starting out (for example, someone like me who has lots of experience as an in-house bookkeeper but isn't established yet as a freelancer), subcontract work to that person, and then check their work carefully before handing it back to the client. That's essentially what a lot of accountants do when they sub bookkeeping work to bookkeepers. Just be sure it's someone who won't try to walk away with your clients.
How much work are you looking to outsource whilst your on maternity leave? would this work been done remotely or at the clients premises? - I'm based in Bristol and might be able to help out you out - or at least maybe we could have a chat about some options you are looking for?